The present invention relates to a method of controlling the heating of an oxygen sensor fitted to the exhaust system of an internal combustion engine for the purpose of controlling air--fuel mixture air/fuel ratio, and to a system for practicing the method. More particularly, the present invention relates to such a method and device for oxygen sensor heating control which provide maximum oxygen sensor heating for a certain time after starting up of the engine from a cold condition, so as to warm up the oxygen sensor to a temperature not less than its minimum active temperature as quickly as possible in order to perform air/fuel ratio control as soon as practicable.
It is known to fit an oxygen sensor to the exhaust system of an internal combustion engine. Such an oxygen sensor typically comprises a solid electrolyte or semiconductor, and varies a generated current or resistance in response to the concentration of oxygen in the exhaust gases of the engine. This electrical signal is fed to a control device which controls the amount of fuel provided to the engine in relation to the amount of air sucked thereinto, and is used for controlling the air/fuel ratio of the air--fuel mixture supplied to the engine by a feedback process. Various such forms of control device, which practice various methods of air--fuel mixture ratio control, are per se known.
The output of the sensor element of such an oxygen sensor varies with temperature, and, particularly when the air/fuel ratio is weak and is in the range of 14.5 to 25, in order for the sensor element to accurately measure the oxygen concentration, said sensor element must be maintained at a temperature higher than a certain critical minimum active temperature. This maintenance of the temperature of the sensor element can be done by using a heater, and oxygen sensors with sensor element heaters have already been proposed, along with methods for operation of such heaters; for example in Japanese Patent Application No. 53-78476, which has been published as Japanese Patent Publication No. 54-13396. Further, in Japanese Patent Application No. 53-83120, which has been published as Japanese Patent Publication No. 54-21393, there has been proposed a method and a system for control of the electrical power supplied to such an oxygen sensor element heater, in which the power is varied as a function of intake manifold pressure, of throttle opening, and of engine revolution speed, so as to ensure that the oxygen sensor element is kept at a temperature no lower than its minimum active temperature.
The sensor element of such an oxygen sensor fitted to an exhaust system is of course at a temperature substantially the same as that of the engine as a whole, when the engine has not been running for any substantial time. After the starting up of the engine it is very desirable for the sensor element to be warmed up at least to its said certain critical minimum active temperature as quickly as possible, in order to be able to properly perform control of the air/fuel ratio of the air--fuel mixture supplied to the engine by the abovementioned type of feedback process as quickly as possible after engine starting up, so as to provide good engine performance and fuel economy while maintaining good quality of the exhaust emissions of the engine; and this quick warming up of the sensor element is particularly required when the initial temperature of the engine and the sensor element is low, as during winter conditions or the like. In more detail, in the case of starting up of the engine from cold, when the coolant temperature of the engine is less than a certain critical value, then controlling the supply of electrical power to the heater according to the engine operational conditions as explained in the previous paragraph delays warming up of the sensor element and consequently delays the time point at which proper air/fuel ratio control of the intake mixture can be performed; while on the other hand, if on engine starting up the coolant temperature of the engine is greater than said certain critical value, then such normal control of the supply of electrical power to the sensor element heater does not cause such a problem.
In Japanese Patent Application No. 56-181006, which has been published as Japanese Patent Publication No. 58-83241, the suggestion has been made to supply a large current to the sensor element heater during engine warming up, so as to heat up the sensor element quickly. However, in this prior art, this sensor element heater power is suggested to be supplied in every instance of starting up, irrespective of whether the engine is cold or warm when it is being started up, and accordingly sometimes such sensor element heater power may be supplied when not necessary. This is in some circumstances wasteful of energy, and may lead to overheating of the sensor element, as well as placing undue strain on the heater element.